Sleep

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Abstract
SLEEP is a behavioral state identified by a characteristic immobile posture and diminished but readily reversible sensitivity to external stimuli. Since most definitions of behavior strongly imply action, the absence of motion and reactivity may account for the omission of sleep from most systematic biology textbooks and even from detailed accounts of natural history. It is as if sleep were thought of as the absence of behavior. Yet, as the ethologist Tinbergen1 has suggested, there are many reasons to class sleep with the instinctive behaviors: it is innate, automatic, exigent and essential; and it is preceded by distinct appetitive behavior . . .
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